Spread 2 tablespoons tomato sauce over half of each tortilla to within 1 inch of edge. Place 6 pepperoni slices and 1/3 cup cheese over sauce on each tortilla. Fold each in half to cover filling.

Spray 12-inch nonstick skillet or large griddle with cooking spray; heat over medium-high heat. Place 2 quesadillas side-by-side in skillet; cook 4 minutes or until each side is browned lightly and cheese melts, turning once. Repeat with remaining quesadillas.

Place remaining tomato sauce in small microwave-safe bowl; cover. Microwave on HIGH 45 seconds; stir. Cut each quesadilla into 3 wedges. Serve with tomato sauce on side.